1. Field of the Invention
The present invention relates to a rotor disc for use in an electrical machine and in particular to the construction of an active rim on the rotor disc.
2. Description of Background Information
Electrical machines, which operate in accordance with transverse flux principles, comprise an armature winding in the form of circular coil co-axial with a rotor. The rotor consists of one or more active rims comprising a multiplicity of magnets and laminated poles, fastened to the disc. The armature winding links the flux generated by the permanent magnets mounted on the rim of the rotor disc by means of a series of stator cores.
The rotor disc may support several rotor rims typically arranged in pairs on opposite sides of the disc. Each rotor rim is circumferential and consists of a single row of magnets and pole pieces.
European patent 0779695-B, in the name of Rolls-Royce Power Engineering plc, describes an arrangement in which the laminations in each pole piece are clamped by one or more bolts. The bolts pass through the lamination stack and the rotor disc. It is necessary for the rotor poles, which are made from a ferromagnetic material such as silicon-iron alloy, to be electrically insulated from the bolts passing through them to minimise power losses due to eddy currents.
Conventional methods of achieving the required insulation include providing an insulating coating on the surface of the bolt or sheathing the bolt with an insulated tube. The application of a coating to the bolt is expensive whilst the sheath may be damaged during installation and requires an additional assembly clearance which adversely affects build accuracy.
The present invention seeks to provide an insulated pole piece, which is not expensive to produce, has adequate insulation integrity and which more accurately locates the pole piece relative to the disc.
According to the present invention a rotor disc for use in an electrical machine has at least one circumferential rotor rim mounted thereon. The rotor rim comprises at least one row of alternate magnets and laminated pole pieces. The laminations in each pole piece are supported by at least one bolt which extends through the rotor disc. A clearance is provided between the laminations and the bolt passing therethrough. The clearance insulates the bolt from the laminations in each pole piece.
Preferably the laminations are mounted concentrically on the bolt in a radially spaced relationship to provide the insulating clearance. This eases assembly of the rotor rim, as a close fit is not required between the laminated pole pieces and the bolt.
The laminations may be bonded together to form a stack, the stack of bonded laminations being mounted concentrically on the bolt in a radially spaced relationship. Bonding the laminations into a stack gives a discrete component that is easier to assemble.
In the preferred embodiment of the present invention the stack of bonded laminations is mounted concentrically on the bolt in a radially spaced relationship by the provision of insulated annular members at either end of the stack. The insulated annular members may be recessed into either end to minimise the insulating clearance over most of the stack length and ease assembly with small radial clearances.
Preferably the insulated annular members are resilient and are formed from an elastomeric material, which allows them to be pressed into the recesses and ensures accurate location of the pole piece relative to the rotor disc.
The laminated pole pieces may be compressed. Preferably resilient means, such as nuts and spring washers, are provided on the bolt for compressing the laminated pole pieces. The nuts and spring washers maintain the correct compressive force on the laminated pole pieces throughout operation.